The present invention relates to an optical frequency converter utilizing a nonlinear optical effect and, more particularly, to an optical frequency converter which uses a ring resonator or a V-shaped resonator composed of a nonlinear optical crystal to stably convert a frequency of a fundamental harmonic of the laser light emitted by a semiconductor laser into a different frequency.
It has been heretofore known that the light of higher harmonics is generated, utilizing a nonlinear optical effect, by causing the light to enter a nonlinear medium or nonlinear optical crystal. For example, if a second-order nonlinear medium which is adapted for second-harmonic generation (SHG) and is not symmetrical about its center is used, it is easy to produce light of frequency 2.omega. simply by making laser of frequency .omega. enter the medium. Therefore, media of this kind are frequently used as optical frequency converters which convert input frequencies (input wavelengths) into higher frequencies (shorter wavelengths). Second harmonics are generated efficiently where phase matching conditions are satisfied. More specifically, if the nonlinear medium of interest has birefringence, and if the crystal orientation and the polarizing characteristics of the optics are such that the refractive index for the second harmonic agrees with the refractive index for the fundamental harmonic (fundamental wave), the phase matching conditions are met. Examples of nonlinear materials which can be used for second-harmonic generation and can be phase-matched are KH.sub.2 PO.sub.4 (KDP), LiNbO.sub.3, KNbO.sub.3, KTP, Ag.sub.3 AsS.sub.3, CO(NH.sub.2).sub.2 (urea), and the like.
An optical frequency converter comprising a ring resonator in which the aforementioned devices for the light of the second-harmonic generation are arranged symmetrically to stably generate the second harmonic has been known. Various types of this converter have been proposed. One kind is described in U S. Pat. No. 4,884,276. In this respect, semiconductor lasers are inferior in light frequency stability (or wavelength stability) to gas lasers. Therefore, it has become increasingly important that the laser light frequency (or the wavelength of the laser light) from a semiconductor laser be stabilized. One method of satisfying this requirement is to use laser light returning from a ring resonator in such a way that the semiconductor laser emits laser light of a stabilized laser light frequency (or wavelength).
High-output gas lasers such as Ar ion lasers are available. However, they are large in size and have low current conversion efficiencies. In addition, their power supply is large and needs to be water-cooled. Consequently, there is a demand for a compact laser light source.
For instance, Seno et al. have proposed an optical frequency converter in Preprint of "Laser and Atomic Oscillators and Ultimate Photon Optics" Symposium, 1990, p. 38. This converter is shown in FIG. 9, and comprises a diode laser 101, a lens 103, a mirror 105 whose position can be varied quite slightly by a PZT (lead zirconate titanate) device 107, two partially transmitting mirrors 109, 113 and a totally reflecting mirror 111 which are arranged in a predetermined manner to form a ring resonator, a crystal device 115 for second-harmonic generation which is placed in one geometrical light path inside the ring resonator, a dichroic mirror 117 which transmits the second harmonic exiting from the ring resonator as the output light and which reflects the laser light of the semiconductor laser leaking from the ring resonator to use the leaking laser light for electrical feedback, a light detector 119 which detects the leaking laser light, that is, the fundamental harmonic, and a signal processing-circuit 121 which processes the output signal from the detector 119 and which activates the PZT device 107 in response to the obtained signal so that the mirror 105 moves a quite small distance for varying the length of the optical path.
In the optical frequency converter described above, a part of the laser light traveling on the resonant mode is reflected by one end surface of the device 115 for second-harmonic generation incorporated in the ring resonator comprising the mirrors 109, 111, and 113 and returned to the diode laser 101. Generally, this part of the laser light is termed the returning (laser) light. If this returning light goes back to the diode laser 101 when the ring resonator is resonating, then the oscillation frequency of the diode laser 101 is automatically locked to the resonant frequency of the returning light. This phenomenon is known as mode locking by optical feedback. As a result, fluctuations of the frequency are suppressed. Consequently, the diode laser 101 emits laser light of a frequency substantially equal to the resonant frequency of the ring resonator. Thus, ring resonator stably produces the second harmonic of the laser light.
In the ring resonator constructed as described above and using the device for second-harmonic generation, two end surfaces produce returning light. That is, the laser light reflection occurs at two locations. Therefore, the resonating laser light is consumed by the returning light. This gives rise to a loss of the laser, thus deteriorating the efficiency at which the laser is converted into the second harmonic. Also, dust in the air tends to adhere to the mirrors, reducing the internal energy of the resonator. The result is that the output is decreased.
As described in the Applied Physics Letters of American Institute of Physics, Vol. 56, No. 23, Jun. 4, 1990, pp. 2291-2292, a monolithic ring resonator is known. With this resonator, however, the frequency of a semiconductor laser must be stabilized by the use of an external electrical signal in order to obtain the second harmonic stably. Furthermore, an optical isolator is needed. For these reasons, this resonator is expensive Moreover, it is impossible to sufficiently regulate the frequency only by the external signal The produced second harmonic is unstable. The ring resonator using the half-mirrors of the above-described construction is about several centimeters square and so it is difficult to miniaturize the resonator. Also, much labor is needed to adjust it. Furthermore, it is susceptible to vibration.